Decrease of microsphere is ischemic tissue over time poses a serious problem to this method of blood flow measurement. Myocardial water content was studied to determine the extent to which edema contributed to the decreased microsphere content. After four hours of occlusion, a 21% decrease in preocclusion microsphere content was noted in the ischemic endocardium. While the same tissue showed a 3% increase in water content, the amount of edema and extent of microsphere decrease were not correlated. Alterations in microsphere content cannot be corrected by measurement of myocardial water content.